The present invention relates to a technique of a loudspeaker system and a loudspeaker enclosure.
As a loudspeaker system for reinforcing a low sound, a loudspeaker system of a bass reflex type has been known. In the loudspeaker system of the bass reflex type, a resonant tube for coupling an internal space and an external space of an enclosure is provided in the enclosure having a loudspeaker unit, and a Helmholtz resonance is utilized to reinforce the low sound. In the case in which a volume of the enclosure is small in the loudspeaker system, however, the resonant tube is to be small-sized and elongated in order to reduce a resonance frequency. Consequently, an air resistance is increased so that a low sound reinforcing function is remarkably deteriorated. Moreover, there is a problem in that a wind noise like a whistle is generated because a speed of air passing through the resonant tube is increased very greatly.
It is also possible to provide a drone cone in place of the resonant tube. In the case in which the drone cone is used, however, it is necessary to increase a mass of the drone cone in order to reduce a resonance frequency. In order to reduce the resonance frequency, it is necessary to increase a compliance of an edge for supporting a diaphragm. In order to support the diaphragm having a great mass, however, a spring property and a strength of the edge are to be increased, which is contrary to the compliance. Moreover, it is hard to vibrate a heavy diaphragm completely in parallel, and an abnormal vibration referred to as rolling or rocking is apt to be generated together. The abnormal vibration increases a distortion and consumes a useless energy, thereby reducing an efficiency.
In order to compensate for the defects of the drone cone, for example, there has been proposed a technique disclosed in WO0032010. According to the method, it is possible to prevent the rolling and the rocking. However, there is employed a structure in which the weight of the diaphragm is supported on the edge provided therearound. For this reason, there is a problem in that a strength is required for the edge and Q of a vibration is reduced by a braking effect.
Therefore, the applicant invented a loudspeaker system shown in FIGS. 16 and 17. In the loudspeaker system, a diaphragm 21 which can be vibrated by an elasticity is provided in a state in which one end is fixed to one surface of a loudspeaker enclosure 20. In one surface provided with the diaphragm 21, furthermore, an opening portion is provided in a position corresponding to a vibrating portion of the diaphragm 21, and there is provided a sealing member 23 for closing a gap 22 formed between the diaphragm 21 and an edge part of the opening portion in a state in which the vibration of the diaphragm 21 is enabled and for holding an airtightness of the loudspeaker enclosure 20. The diaphragm 21 thus constituted will be referred to as a passive diaphragm.
In the loudspeaker system, when a speaker unit 10 is driven, a vibration of a cone paper of the speaker unit 10 is propagated to air in the loudspeaker enclosure 20 and the diaphragm 21 is vibrated by the vibration of the air. At this time, the diaphragm 21 to be vibrated in a state in which the sealing member 23 is caused to hold the airtightness reduces or increases an air volume in the loudspeaker enclosure 20 when it is vibrated. Accordingly, a new resonance frequency is generated between a compliance (a mechanical flexibility) to which an air spring of the loudspeaker enclosure 20 is also applied in addition to the elasticity of the diaphragm 21 and an equivalent mass of the diaphragm 21. As a result, there is generated a sound to be reproduced around the resonance frequency of the diaphragm 21. The resonance frequency can easily be set to have a desirable value in a low sound region.
However, the loudspeaker system has the following drawbacks. The diaphragm 21 is wholly bent like a fan and carries out a vibration in a primary mode as shown in FIG. 18A. At the same time, there are generated vibrations in higher order modes, for example, a secondary mode shown in FIG. 18B and a tertiary mode shown in FIG. 18C. When the vibrations in the secondary mode and the tertiary mode are generated, an acoustic wave generated from each portion of the diaphragm 21 is cancelled so that an acoustic converting efficiency is reduced because phases of the vibrations are different from each other in the respective portions of the diaphragm 21.
In general, it is possible to generate the vibration in the primary mode most greatly by properly setting a rigidity of the diaphragm 21. In the case in which the loudspeaker enclosure 20 is designed to be elongated, however, the diaphragm 21 is also elongated. In the case in which the diaphragm 21 having a flexibility which can carry out the vibration by the bending is set to be exactly elongated, an apparent bending rigidity in a longitudinal direction is reduced and a component of a higher order vibration is thus increased so that an excellent low sound cannot be reproduced.